1. Field of the Invention
The present invention relates to an image forming apparatus equipped with a sheet feeding apparatus configured to feed a sheet to an image forming unit.
2. Description of the Related Art
Conventionally, an image forming apparatus such as a printer or a copying machine is equipped with a sheet feeding apparatus for supplying sheets one by one to an image forming unit from a tray on which the sheets are stacked.
Generally speaking, between the sheet feeding apparatus and a registration roller pair for correcting skew feed and feeding out a sheet in synchronization with an image to be formed in the image forming unit, there is arranged a draw roller for conveying the sheet fed from the sheet feeding apparatus. However, to achieve a reduction in the size of the apparatus as a whole, there has been proposed a construction in which the registration roller pair is arranged immediately downstream of the sheet feeding apparatus without providing any draw roller. This technique is discussed in Japanese Patent Application Laid-Open No. 11-343050.
FIG. 9 illustrates an example of this sheet feeding apparatus. In FIG. 9, a sheet feeding apparatus has a sheet feeding tray 71, on which sheets S are stacked, and a vertically rotatable pressurization plate 70 provided at the forward end of the sheet feeding tray 71 is held down by a cam (not illustrated) against the elastic force of springs 72a and 72b. 
When the sheet feeding apparatus receives a feeding signal from a control unit (not illustrated), the cam rotates, and the pressurization plate 70 is rotated upwardly in FIG. 9 by the springs 72a and 72b, and the leading edge of the sheet bundle is brought into press contact with a sheet feeding roller 51. In this state, the sheet feeding roller 51 is rotated, thereby feeding the uppermost sheet.
When a plurality of sheets are fed in an overlapped state, the sheets are separated from each other by a separation roller 53 held in press contact with the sheet feeding roller 51 by a spring 56.
Here, after a sheet has reached a nip portion (press contact portion) formed by the sheet feeding roller 51 and the separation roller 53, a pressurization plate 70 is rotated downwards in FIG. 9 by a cam (not illustrated), thereby separating the stacked sheets S and the sheet feeding roller 51 from each other.
As a result, it is possible to effectively separate double-fed sheets from each other. In other words, if the sheets on the pressurization plate 70 are kept in press contact with the sheet feeding roller 51, the leading end of the stacked sheet bundle is brought into press contact with the sheet feeding roller 51 and the pressurization plate 70, so that the next sheet may be drawn out due to the frictional force between sheets.
In view of this, the pressurization plate 70 is rotated downward, whereby the stacked sheet bundle is not pressed against the sheet feeding roller 51, and it is possible to reduce the possibility of the next sheet being drawn out due to frictional force, thus making it possible to feed the sheets in a state in which they are reliably separated from each other.
On the downstream in the sheet conveyance direction of the sheet feeding apparatus, there is provided a registration roller pair 57 for feeding a sheet to an image forming unit in a synchronized manner to correct skew feed of the sheet and to effect registration between the sheet and an image formed in the image forming unit (not illustrated).
The sheet fed by the sheet feeding roller 51 is conveyed with its leading edge directed to the nip portion of the registration roller pair 57 at rest, and the leading edge abuts the nip portion to form a loop of a predetermined amount in the sheet, whereby skew feed of the sheet is corrected. And, the registration roller pair 57 is rotated by an image leading edge synchronous signal issued from a control unit (not illustrated), whereby the sheet enters the nip portion of the registration roller pair 57 to be conveyed toward the image forming unit.
In the sheet feeding apparatus illustrated in FIG. 9, after a loop of a predetermined amount has been formed in the sheet between the sheet feeding roller 51 and the registration roller pair 57, if an attempt is made to cause the sheet to enter the nip portion through rotation of the registration roller pair 57, the sheet sometimes may be prevented from entering the nip portion.
This may be due to the fact that, when the sheet is caused to abut the nip portion of the registration roller pair 57 to form a loop, the separation roller 53 is pushed down in the FIG. 9 away from the sheet feeding roller 51 due to the rigidity of the sheet. More specifically, the separation roller 53 is pushed down by the force restoring the looped and deflected sheet to the former state, whereby the press contact force between the sheet and the sheet feeding roller 51 is weakened, with the result that slippage occurs between the sheet feeding roller 51 and the sheet, making it impossible to obtain the requisite conveyance force.
In particular, this is more likely to occur in the case where a sheet of high rigidity such as an envelope is conveyed since the force with which the separation roller 53 is pushed down is then further increased.